Conventionally, various methods of removing noise of a flat part while maintaining an edge by evaluating the degree of local unevenness of an image signal to weaken and strengthen noise reduction in accordance with an evaluation result when the noise of the image signal is removed have been proposed.
For example, PTL 1 discloses that the intensity of local edges in a plurality of directions around a target pixel is examined, so that flatness indicating whether the periphery of the target pixel is flat or has a strong edge is calculated, and a high frequency coring threshold value is controlled in accordance with the calculated flatness. In the technology disclosed in PTL 1, in a case where a range in which the intensity of an edge is examined is limited to peripheral pixels around a target pixel, an influence of noise becomes greater.
PTL 2 discloses a technology of obtaining the local unevenness (dispersion value) of an image signal, and discriminating between a flat part and an edge part or a texture part on the basis of a result obtained by smoothing the obtained local unevenness. In a case of the technology disclosed in PTL 2, the unevenness of a low frequency due to noise appears in the determination result by smoothing. When such a determination result is used for a noise reduction process, particularly in a low contrast texture part, large scale nonuniformity of texture reproduction occurs because of the low frequency of the determination result.
PTL 3 discloses a technology of adjusting the intensity of noise reduction on the basis of the ratio of a smoothed dispersion value obtained by smoothing a local dispersion value and a local dispersion value. In the technology disclosed in PTL 3, in a part where the local dispersion value is larger than the smoothed dispersion value, the intensity of noise reduction is weak, and in a part where the local dispersion value is smaller than the smoothed dispersion value, the intensity of noise reduction is strong. However, in the part where the local dispersion value is larger than the smoothed dispersion value, the structure is likely to remain during the noise reduction process, and in the part where the local dispersion value is smaller than the smoothed dispersion value, the structure is originally unlikely to relatively remain in the noise reduction process. Therefore, when control of weakening the noise reduction intensity of the former, and strengthening the noise reduction intensity of the latter is performed, a difference between a part where the structure remains and a part where the structure is broken in a low contrast part becomes large.